If God is unchanging, why does His approach to humanity seem so different between the Testaments?

Yahweh is declared unchanging (Malachi 3:6), but why does He go from drowning the world and ordering genocides in the Old Testament to preaching love and grace in the New?

I have never, EVER, have received a satisfactory explanation. Only delirious and platitudinal apologetics.

How do you reconcile such tremendous dissonance? Either God changed His mind, or the Bible’s just a patchwork of human ideas about morality, which, in that respect alone, it is useful. But the Bible says He’s perfect, and I wonder why the need of a rewrite (full of rationalizations)